A high proportion of vehicle engines are fitted with turbochargers to improve performance as well as fuel efficiency and emissions levels. Engines which are fitted with turbochargers often incorporate an exhaust gas recirculation (EGR) system, which further reduces emissions values by recirculating a portion of the exhaust gases back to the inlet of the engine. In a low pressure EGR (LP-EGR) system the exhaust gases are reintroduced upstream of the turbocharger compressor inlet. The pressure at this location is low, even in high engine boost conditions, which allows for the low pressure recirculation of the exhaust gases. In contrast, in a high pressure EGR (HP-EGR) system the exhaust gases are reintroduced downstream of the turbocharger compressor outlet and hence exhaust gases must be recirculated at a higher pressure. Some vehicles are fitted with twin turbochargers, which work in series to increase the pressure of inlet gases and recirculated exhaust gases. High pressure EGR gases may be reintroduced upstream or downstream of the second compressor. In order to control the flow of EGR gases, the EGR system may have one or more EGR valves configured to control the reintroduction of the EGR gases back to the inlet of the engine.
When an engine equipped with an EGR system is operating at or close to full load, for example maximum torque output, the EGR valve will typically be closed, reducing the EGR flow to zero. When the EGR valve is closed, condensation can form upstream of the valve as the EGR gases and system cool. When the torque demand on the engine is reduced, the EGR valve opens to reintroduce the EGR gases back to the inlet of the engine. As a result, the condensate that has accumulated upstream of the EGR valve is discharged through the valve and into the inlet of the engine. Such a discharge of condensate is undesirable, especially for an LP EGR system as a large quantity of condensate can damage the compressor of the turbocharger.
One solution is to periodically open the EGR valve, for example for 3 seconds every 30 seconds, to evaporate and discharge the accumulated condensation and to reheat the EGR system, which helps to reduce the rate of condensation. However, at or near full engine load, opening of the EGR valve reduces the air available for combustion and thus results in a torque disturbance which is noticeable by the driver.